UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a contest where the techniques didn't complement each other, with frequent clinching marring the fight.
Two judges had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best performance but what defines champions? They prevail," the victor commented in the arena as he was met by boos from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts felt Parker, thirty-one, merited the victory but the British boxer endured the second loss of his 28-bout professional journey.
Buatsi Secures Win in Slow Encounter
Buatsi's progress has stalled in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to achieve expectations - maybe in what feels like a final roll of the dice - under recent support.
He started carefully, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who worked behind a tight guard to barely take the cleaner stanzas.
The crowd's patience wore thin during regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were difficult to score. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Arthur beat Rea by split-decision in a excellent all-Manchester battle to capture the continental championship.
The venue fell silent for a deeply poignant homage ahead of the contest as 10 bells sounded in honor of local hero Ricky Hatton, who died aged 46 in September.
It was the first major televised boxing event in the city since Hatton's death. As the final chime faded, the stillness became a shout of "there's just one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner was floored by a left hook directly to the jaw in the early round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged soon, noticeable damage forming under his face as the pair landed punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with judges scorecards of draw, 115-113 and 115-112, but both fighters were relentless and courageous, everything their idol Hatton stood for.
